1. Field of the Invention
The present invention relates to a method for writing servo onto disks of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGAs are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or servo sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each servo sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and used to position the head 7 relative to the track.
The fields 1-5 must be written onto the disk surfaces during the manufacturing process of the disk drive. These fields are typically written with a servo writer. The servo tracks are sometimes written using a number of spiral servo tracks initially written onto the disks. FIG. 2 shows an example of a number of spiral servo tracks written onto a disk. Using spiral servo tracks is sometimes referred to as an Ammonite servo write process. The spiral servo tracks are used to write the final radial servo tracks that are utilized during the normal operation of the disk drive. This process is described in U.S. Pat. No. 5,668,679 issued to Swearingen et al.
As shown in FIG. 3, the disk has a plurality of spiral servo patterns incrementally spaced across the surface of the disk. To write servo the servo writer reads the disk to detect a spiral servo signal. The servo writer uses the detected spiral servo signal to time the writing of a permanent A, B, C and D servo burst pattern.
The final (radial) servo patterns are written utilizing a position error signal (“PES”) generated from the spiral servo signals. The mechanical system of the drive and servo writer limit the system's ability to rapidly respond to excursions in the position of the writer during the writing process. As a result, such excursions typically extend over many sectors, comprising a significant fraction of a revolution. Such excursions lead to errors in the servo patterns' written position.
Errors in writing the servo patterns are typically minimized by repeatedly writing the patterns on the same track. The final position of each servo pattern of the disk track represents the maximum excursion during all writes to that pattern. (The variation in the maximum of several random values is less than the variation in an individual value.)
Because successive servo patterns are written in sequence, residual variations in the final position from one servo sector to the next are also minimized, except for possible discontinuities at the beginning or end of a writing sequence. It would be desirable to provide a technique that can avoid these discontinuities as well, and that can rapidly respond to excursions, for example within a small fraction of a revolution of the disk.